When one solid-state laser is used to pump another solid-state laser, it is important, for efficient operation, that essentially all the pump photons be absorbed by the second laser material. This is conventionally achieved by making the laser material long enough such that more than 90% of the light at the pump wavelength is absorbed in a single pass through the material. This is not always practical with weakly absorbing materials (i.e., those materials where the absorption is only a few percent per centimeter). Sufficiently large pieces of the laser material may be unobtainable of the losses introduced by the long laser rod may be unacceptable. Simple techniques for increasing the material absorption (e.g., increasing the active ion concentration) are usually not feasible, since they usually have unacceptable physical consequences, such as reductions in the excited state lifetime. Thus, alternative techniques for efficiently pumping these weakly absorbing materials are desirable.
The potential medical uses for 3 .mu.m lasers has spurred a search for efficient high power solid-state sources. For example, holmium laser operation has been demonstrated using flashlamp pumping in a number of different hosts. The lowest thresholds have been observed using holmium doped yttrium aluminum perovskite (YAlO.sub.3) or Ho:YAP. Emission in Ho:YAP has been reported at 2.92 .mu.m and 3.02 .mu.m on the .sup.5 I.sub.6 .fwdarw..sup.5 I.sub.7 transition.
Intracavity laser pumping, using the 1.08 .mu.m line of a Nd:YAP laser to pump a Ho:YAP rod has been reported. S. R. Bowman, W. S. Rabinovich, A. P. Bowman and B. J. Feldman, "Laser Pumped 3 .mu.m Ho:YAlO.sub.3 Laser, " Annual Meeting of Optical Society of America, Session TU04, Oct. 16, 1989. Specifically, a Ho:YAP rod was placed inside the cavity of a flashlamp pumped Nd:YAP laser whose cavity losses, aside from the rod, were low. Since the rod was the dominant loss in the Nd:YAP cavity, it absorbed most of the pump light after many passes. A separate cavity, in and oriented at a slight angle from the axis of the 1.08 .mu.m Nd:YAP laser cavity, was used to contain the Ho:YAP rod. Emission was observed at 2.92 .mu.m as well as at a new line at 2.85 .mu.m. The 2.85 .mu.m line was extremely sensitive to atmospheric water absorption and rankle only when the laser was purged with dry nitrogen. In a nonoptimized cavity, slope efficiencies of about 4% were observed relative to the 1.08 .mu.m pump and a maximum output energy of 8 mJ was observed.
The physical size of the Bowman et al. system is relatively large (e.g., a 6.3 mm by 75 mm holmium doped laser rod). High power densities were required and, because of the geometry, the Ho:YAP host material was not used efficiently. The thresholds were also relatively high.
Erbium is a material of practical interest since it can be used to generate optical radiation at 1.54 .mu.m which is well into the "eye safe" region. An Yb,Er:glass laser, pumped by a flash lamp, has an energy efficiency in the range of 0.3 to 0.5 percent. By contrast, energy conversion efficiencies of erbium pumped by ytterbium, under optimum conditions, have been predicted to be in the range of 40 to 50 percent. Snitzer & Woodcock, Applied Physics Letters, Vol. 6, page 45 (1965). Glasses sensitized with Yb and Er and pumped by a Nd laser near 1060 nm have been studied by Gapontsev et al. and good efficiencies have been observed. Optics Comm. 46 (1983) 226. Pumping of Yb using a 860 nm diode in a fiber laser has been studied by W. L. Barnes et al. "Er.sup.3+ -Yb.sup.3+ and Er.sup.3+ Doped Fiber Lasers", Journal of Lightwave Tech., Vol. 7, No. 10, Oct. 1989 at page 1461.
A barium-alumo-metaphosphate glass, activated with 1.5.times.1021 cm.sup.-3 Yb.sup.3+ ions and 2.5.times.10.sup.19 cm.sup.-3 Er.sup.3+ ions has also been studied. "Effective 1.054-1.54 .mu. Stimulated Emission Conversion", V. P. Gapontsev et al., JETP Letters (1973) 18, p. 251-253. In that study a generally rectangular active Yb.sup.3+ -Er.sup.3+ element was used. It had a cross section of 10 mm by 14 mm and a length of 70 mm with two opposite faces inclined at 45 degrees. It was placed in a resonator comprising two active elements of LGS-40 phosphate glass in the form of generally rectangular plates which had a 10 mm by 32 mm cross section, a length of 280 mm and faces cut at the Brewster angle. The two phosphate glass elements were symmetrically arranged relative to the Yb.sup.3+ -Er.sup.3+ element.
It would be desirable to use laser materials which are less strongly absorbing and/or much thinner than those used in a conventional extracavity system. If efficient pumping could be achieved in an intracavity geometry, many potentially valuable materials would become available for use and many of the problems associated with physically large systems would be reduced. Moreover, it would be possible to exploit the low power benefits of laser diodes.